1. Field of the Invention
The invention relates to a vehicle powertrain having at least one torque flow path defined by an infinitely variable transmission.
2. Background Art
Heavy-duty powertrains with fixed ratio gearing, such as powertrains used for heavy-duty trucks, use an engine compression braking technique. Such trucks typically are equipped with wheel friction brakes to complement the compression braking of the engine, but engine compression braking alone ideally would be used by a trucker at moderate and high road speeds. Engine compression braking is less effective at lower road speeds.
Braking horsepower is developed by the friction of the engine and by compression energy developed by reciprocating pistons of the engine. During operation at high engine speed, the frictional braking horsepower is increased, as well as the compression energy. In those instances when the engine is operating with one or more cylinders disabled, the total braking horsepower can be increased further by enabling selectively the engine cylinders. Braking horsepower increases as each cylinder is enabled. To augment the braking horsepower, an engine cooling fan may be enabled to provide still more braking horsepower. As in the case of engine braking horsepower, cooling fan braking horsepower increases with increased engine speed.
Since the total engine braking horsepower is a function of engine speed, a geared transmission with discrete ratios will provide little flexibility in controlling the engine speed. This makes it difficult to provide adequate control of engine braking horsepower.